Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200701

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
101 AM CST Mon Nov 20 2017

100 AM CST

Through tonight...

High pressure is across the Ohio/Tennessee valleys early this
morning, with low pressure digging southeast across the Canadian
prairies. The high will only slowly shift to the mid Atlantic
today, while the low will deepen as it moves toward Lake Superior.
The upper air pattern with WNW flow will support some cirrus
drifting through that will thin out with time today, otherwise the
weather story of the day will be the mild conditions and breezy
southwest winds that result from the increased gradient between
the two surface features. There is a pretty strong inversion which
will limit mixing, but winds will be potent enough not far off
the ground to support gusts to 30-35 mph this afternoon. Look for
temperatures to peak near 50 or so. The low will continue through
Canada just to the north of Lake Superior, therefore breezy
southwest winds will continue, and with some increasing clouds
overnight lows will hold in the mid to even upper 30s.



100 AM CST

Tuesday through Sunday...

While the forecast largely remains dry, the temperature roller
coaster ride will continue to be the story in the long term. A
sharp positively tilted shortwave trough will shift through
Wisconsin, and will steer a decent cold front through our region
through the day. Models do not paint much precip with the front
other maybe some isolated light showers or sprinkles, with better
(though still lower) chances across northwest/north central
Indiana in the afternoon. Breezy southwest winds do make a sharp
shift to the northwest and we get another shot of cold air. Cold
low in the 20s Tuesday night give way to highs again only in the
mid/upper 30s Wednesday. Winds will slacken through the day
Wednesday as high pressure returns to the mid Missouri valley,
with the ridge axis extending across northeast IL/northwest

The pattern from earlier in the week repeats itself for
Thanksgiving as the high will shift to the Tennessee valley on
the Thanksgiving holiday while low pressure will return to the
Canadian prairies. It should be a decent day with sunshine/high
clouds, and highs will be a tad below normal, but winds will be
relatively light. Then the breezy southwest winds kick up for
Friday as the high will cruise off to the mid Atlantic and the
Canadian low follows a similar path north of Lake Superior on
Friday. This time the low is deeper and bit closer to the region,
so we may see some more clouds ahead of it. Low level temps are
even warmer than what we expect for today, so it will again be
mild with highs in the 50 ballpark. There are some low precip
chances with the associated cold front. Temps again take a tumble
for the weekend.



For the 06Z TAFs...

Dry weather will prevail over the TAFs through Monday but there
will be gusty winds of note by Monday afternoon persisting into
Monday night, and low-level wind shear (LLWS) potential Monday

As high pressure pulls away through daybreak, speeds will start
to increase. Frequent gusts are expected by mid to late morning.
The winds will steadily increase through the afternoon with the
sustained showing no diminishing after dark Monday evening and
possibly even increasing. Confidence in the wind trends through
Monday is medium-high, and in specific gust magnitude is medium
(low on frequency of gusts after dark). The wind direction within
200-220 degrees has medium-high confidence.

On Monday evening, a southwest low-level jet of 50-55 kt is
expected to develop with a base somewhere between 1000-2000 ft.
Given that surface winds will be breezy on Monday evening, there
is uncertainty on whether the rate of increase in wind with
height will reach true low-level wind shear criteria. There is
likely to be an inversion however, and conceptually that is often
where a sharp increase in speed can happen. So thought it worth
including in the TAFs.



212 PM CST

Surface ridge axis has been moving across the region, with some
slight diminishing wind trend today. However, don`t anticipate any
additional weakening as the start of the week will once again
remain windy. Gradient picks back up ahead of an approaching low
to the northwest. Speeds will initially pick up over the southern
portions of the lake tonight into Monday, but anticipate this
increasing trend to include the northern portions later in the day
Monday. By late Monday into Monday evening, will see gales
develop over the entire open waters and because of this, have
issued a gale watch. Some lower confidence remains with start and
end times to these gales, and will need to monitor this for a
likely later transition to a warning. Also to monitor, the
likelihood for high end gales to 45 kt and even the slight
possibility for a brief window of storm force winds. Hazardous
conditions for small craft will expand on Monday and likely
continue into mid week. There is the possibility for gales to be
observed for a time Monday night into Tuesday morning. However,
confidence is low at this time. Will include mention of gale force
gusts during this time, and continue to monitor the possibility
of more prevailing gales over the nearshore waters.



LM...Gale Warning...LMZ777-LMZ779...3 PM Monday to 3 PM Tuesday.

     Small Craft Advisory...IN nearshore waters until 9 AM Wednesday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...noon Monday to 6
     PM Tuesday.




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